diff --git a/tests/domain/modelling/test_roof_recommendation.py b/tests/domain/modelling/test_roof_recommendation.py
index 3acdbb86f..77e9c0124 100644
--- a/tests/domain/modelling/test_roof_recommendation.py
+++ b/tests/domain/modelling/test_roof_recommendation.py
@@ -167,6 +167,30 @@ def test_vaulted_ceiling_with_measured_thickness_yields_no_recommendation() -> N
     assert recommendation is None
 
 
+def test_loft_roof_with_nd_thickness_on_pre_1950_dwelling_yields_loft_insulation() -> None:
+    # Arrange — the loft fallback shares the sentinel semantics (ADR-0047):
+    # "ND" on an age-band-C pitched loft resolves to as-built uninsulated, so
+    # the joist measure fires just as it does for an explicit 0.
+    baseline: EpcPropertyData = build_epc()
+    main: SapBuildingPart = _part(baseline, BuildingPartIdentifier.MAIN)
+    main.roof_insulation_thickness = "ND"
+    main.construction_age_band = "C"
+
+    # Act
+    recommendation: Recommendation | None = recommend_roof_insulation(
+        baseline, _StubProducts()
+    )
+
+    # Assert
+    assert recommendation is not None
+    option = recommendation.options[0]
+    assert option.measure_type == "loft_insulation"
+    simulated: EpcPropertyData = apply_simulations(baseline, [option.overlay])
+    assert (
+        _part(simulated, BuildingPartIdentifier.MAIN).roof_insulation_thickness == 300
+    )
+
+
 def test_loft_option_carries_cost_from_roof_area_and_product() -> None:
     # Arrange
     baseline: EpcPropertyData = build_epc()  # MAIN roof area 14.85 m^2